The present invention relates generally to packaging for semiconductor integrated circuits and, more particularly, to a process for manufacturing an integrated circuit lead frame structure having selective silver plating thereon.
In the manufacture of integrated circuit devices, lead frames are used to provide electrical interconnection to a semiconductor circuit. Typically, the base metal of the lead frame is copper because of its high thermal conductivity. Other base metals include stainless steel and Alloy 42, which is an alloy of 42% nickel and 58% iron. Heat transfer, rather than electrical conductivity, is of paramount importance in the selection of the lead frame base metal.
It is known to plate the entire surface of a lead frame with a highly-conductive coating in order to improve its electrical conductivity. One such conductive coating is silver, which displays excellent electro-conductivity and corrosion resistance, but is costly because it is a precious metal. There/ore, it is desired that a silver plating should be applied only where its electrical characteristics are particularly needed. In addition, from an economic standpoint, it is desired that the plating should be as thin as possible. In the case of a lead frame, it is desired to limit the area coated with silver to the mounting site where the silicon device is attached, and to the regions where gold wires are attached using, e.g., wire-bonding techniques.
However, using the selective silver plating processes for lead frames which are currently known, yield losses due to mechanical damage to the lead tips are excessive. This is a result of the additional handing of the delicate structure during the plating process. Furthermore, the yield loss is aggravated as the pin count of the lead frame increases and the thickness of the lead frame material decreases.
In view of the above, it is clear that there exists a need to develop a method for providing selective silver plating of a lead frame device which reduces the possibility of mechanical damage and the resultant yield loss which is an improvement over what is currently known in the art.